1. Field of the Invention
The present invention relates to a chuck mechanism provided at a tip end of a striking tool, such as a hammer or a hammer drill, where a bit is insertedly mounted.
2. Description of the Related Art
In a striking tool such as a hammer or a hammer drill, a chuck mechanism for insertedly mounting a bit is provided at the tip end of a housing. As shown in FIG. 4 of Japanese Examined Patent Application Publication No. 3-43003, a conventional chuck mechanism includes a ball movable in a radial direction in a through hole formed in a cylindrical tool holder in which a bit is inserted, and further externally mounts a sliding sleeve for operation, which houses a support ring for pressing the ball to the axis side, a thin-sheet ring which is in contact with the ball and the sliding sleeve, and a coil spring which forces the thin-sheet ring frontward, at the periphery of the tool holder. In normal state of the chuck mechanism, the ball is pressed to the axis side in front of the through hole by the support ring under the force of the coil spring, projects into the tool holder and fits in a groove formed in the axial direction at the outer periphery of the bit, so that the bit does not come off. When the sliding sleeve is slid rearward and thereby the support ring is retracted rearward from the outer periphery side of the ball, the ball moves to the outer periphery side to remove the bit.
In the above-described chuck mechanism, when the bit is inserted into the tool holder, the ball rolled rearward in the through hole by the rear end of the bit. Then the ball is pushed in between the support ring and the thin-sheet ring, by which the ball is moved to the outer periphery side of the tool holder. When the bit is completely inserted, the ball which has been advanced under the force of the coil spring is projected again to the axis side by press of the support ring, and fits in the groove. Thereby, the bit can be prevented from coming off by the ball movement with only insertion of the bit and it is not necessary to retract the sliding sleeve. Therefore, the operability of attachment and detachment of bit is improved. However, only one ball can be used in terms of the construction because if two balls are used where the bit is inserted in the tool being directed upward, the support ring retracts as well when the thin-sheet ring retracts due to the insertion of bit, so that the ball cannot be retracted. For this reason, a force of the ball to prevent the bit from coming off becomes relatively small. Therefore, the ball may come out from the groove of the bit by impact at the time of striking, or the bit or ball may be damaged, which leads to the degradation of reliability and durability.